The invention relates to a method for electronically scanning a light integrating photodiodes, and in particular to a method for electronically scanning one or more photodiode arrays each of which is time shared by a plurality of light beams.
U.S. Pat. No. 5,002,392, the disclosure of which is incorporated herein by reference, discloses a multi-channel optical monitoring system in which a plurality of photodiode arrays are illuminated by light beams projected onto the arrays by way of a shutter mechanism. Groups of individual beams are projected onto respective ones of the arrays, and within each group of beams, the beams are sequentially projected onto an array, so that each array is time shared by a group of beams.
Each of the light beams constitutes an optical channel and passes through a reaction well of a cuvette and then through a transmitting diffraction grating for the purpose of performing an optical analysis of a reaction volume in the reaction well. The photodiode arrays each develop electrical signals corresponding to the spectral distribution of the respective beams falling on the arrays, and the arrays are periodically read by electronic scanning circuitry.
The shutter mechanism disclosed in the aforementioned patent comprises a rotating shutter which includes a number of cam elements mounted on a motor driven shaft. The light beams are incident on the cams in a direction parallel to the rotational planes of the cams. Each cam element is aligned in a respective one of the optical channels and has a cut-out segment greater than 180 so that each cam will block the light beam that it is aligned with for a certain portion of the rotation and will pass the beam for a remaining portion of the rotation. The cut-out segments of the cams are angularly arranged relative to one another so that the rotating shutter sequentially passes the beams in a predetermined sequence.
Each cam thus constitutes a shutter element which opens and closes an optical path during each revolution. Actually, each revolution of a cam may be divided into four periods. A first period occurs during a portion of a turn when the cam completely blocks the optical path of the light beam. A second period occurs when an edge of the cam bordering on the cut-out segment passes through the optical path of the beam during which the optical beam is partially transmitted onto the photodiode array. This is the opening transition. A third period occurs at the conclusion of the opening transition when the cut-out segment of the cam is positioned so that the optical path of the beam is uninterrupted by the cam and therefore the entire beam is fully projected onto the photodiode array. The fourth period is the closing transition period when the other edge of the cut-out segment passes through the optical path of the beam so that the optical beam is again only partially transmitted onto the photodiode array. At the conclusion of the closing transition period the optical beam is totally blocked so that the optical channel enters into a dark period (the first period described above) until the next opening transition period.
The electronic scanning circuitry disclosed in U.S. Pat. No. 5,002,392 involves a charge storage mode of operation whereby each photodiode element integrates light projected thereon by virtue of an electron depletion of its p-n junction which is replenished at the time of scanning. The amount of charge required to replenish the electron depletion is a measure of the integrated light. The charge coupled mode of operation for electronically scanning a photodiode array is well known as disclosed in U.S. Pat. No. 5,002,392 and the prior art cited therein.
The charge storage mode of operation for scanning the photodiode arrays is desirable in an environment in which there are hundreds of low level optical signals that must be evaluated at high speeds and is economical in terms of cost and space since only a single charge coupled amplifier is required in lieu of a separate amplifier for each photodiode element.
In the course of developing a machine utilizing the electronic scanning disclosed in U.S. Pat. No. 5,002,392, a number of practical problems evolved. For example, the light on a photodiode array was integrated during the entire time each rotating shutter element was open, including the opening and closing transition times. Theoretically, the opening and closing transition times should be consistent from revolution to revolution, however, in practice it was found that a slight amount of motor jitter introduced jitter into the optical signal from scan to scan. That is, as a result of motor jitter in the motor driving the rotating shutter, the opening and closing transition times would vary from revolution to revolution, resulting in a different amount of light being integrated from scan to scan, with the characteristics of the optical path otherwise being unchanged. Variation in the integration time from scan to scan thus introduced an error into the signal that was being measured, making it difficult to precisely determine what was happening in the reaction volume of the cuvette.
It is was further discovered that in connection with the time sharing of the photodiode arrays with a plurality of optical channels, there were residual effects from brightly lit previous channels observed in succeeding channels, thus further clouding the measured signal.
It was also found that the intensity of light in the outer light channels was not as great as the intensity of light in the central light channels. However, it is desirable that the light channels be balanced in order to stay within the gain limits of the amplifiers of the scanning circuitry.
Another practical problem that developed concerned the amount of time that was available for the host computer to collect data and calculate gain values for the next scan. Specifically, the data collected during each scan is stored in a buffer memory prior to being forwarded to the host computer. In that no scan data can be stored in the buffer memory at the same time data is being acquired from the buffer by the host computer, and concomitantly a finite period of time is required to empty the buffer register, it is important that sufficient data acquisition time, i.e. the time required to empty the buffer register into the memory of the host computer, be available in order that all scan data stored in buffer memory be utilized by the host computer. It is thus desirable to optimize the scanning sequence in order to provide as much data acquisition time as reasonably possible.